Transferring radiation between a fiber and a second optical element, e.g. an optical circuit often is done by a grating coupler. In order to optimize the functionality of an optical system, one strives for a coupling efficiency that is as high as possible. The coupling efficiency of a grating coupler (e.g. to a fiber) is determined by the amount of power scattered by the grating and the overlap between the Gaussian fiber mode and the scattered power profile by the grating e.g. an exponentially decaying profile for a uniform grating.
The parameters such as e.g. etch depth, filling factor, number of periods N of standard grating couplers can be optimized in order to match the optimum for both criteria. This exercise results in an optimal coupling strength of the grating. The experimental coupling efficiency for standard 1-dimensional or 2-dimensional grating couplers in SOI is typically 30-40%, and the 1 dB optical bandwidth is typically 45 nm.
This optical bandwidth is mainly determined by the number of grating periods N (and therefore the length of the grating). Typically, the number of periods for an SOI-based grating is N=20, resulting in a grating length of the order of the dimensions of the fiber mode. One way to increase the optical bandwidth is by decreasing the number of periods of the grating. In that case, the coupling strength of the grating does not change but less power is scattered to the fiber, since there are less scattering elements. By increasing the coupling strength of the grating (e.g. through deeper etching), the amount of scattered power can be kept substantially identical, but the overlap between the scattered profile and the fiber mode may be very low, since the power is radiated over a too short length. Decreasing the number of periods will thus result in higher optical bandwidth but lower coupling efficiency.